More than fifty percent of the remaining global hydrocarbon resource is heavy oil and bitumen, which is a subset of unconventional hydrocarbon. Indeed, there are an estimated 942 billion barrels of conventional oil of which about 609 billion barrels (i.e., about 66%) are in the Middle East, and 85% of which are in the eastern hemisphere. The estimated recoverable reserve of heavy oil is about 434 billion barrels and there are about 651 billion barrels of bitumen. Of the total recoverable reserve of these two unconventional hydrocarbon sources, about 70% (equivalent to about 832 billion barrels of oil) reside in the western hemisphere; 81% of the bitumen is within North America; and 62% percent of the heavy oil is in South America. The total world oil reserve, including heavy oil, is about 2 trillion barrels which, at a consumption rate of 100 million barrels per day, has an estimated life of about 54 years.
According to the United States Geological Survey (USGS), heavy oil can be categorized according to the density and viscosity of the fluid. The definitions, listed in Table 1 shown below, were obtained from http://pubs.usgs.gov/fs/fs070-03/fs070-03.html. As can be seen in Table 1, the range of fluid viscosity defines the fluid and, ultimately, when combined with the formation permeability, determines fluid mobility.
TABLE 1Definition of the type of heavy oil based on the density ρ,(API gravity) and viscosity η, of the fluid along with commentsconcerning mobility and current extraction methodsDefinitionρ/kg · m−3APIη/mPa · sCommentsMedium heavy oil903 to 946  25 to 18100 to 10Fluid mobile at reservoirconditionsExtra heavy oil933 to 102120 to 710 000 to 100  Fluid not mobile atreservoir conditionsTar sands and985 to 102112 to 7>10 000Not a fluid at reservoirbitumenconditionsOil shaleFluid not mobile atreservoir conditions
In addition to the USGS, the United Nations Information Centre for Heavy Crude and Tar Sands offers a definition for bitumen as petroleum having a viscosity >10,000 mPa's (10,000 cP), while petroleum with viscosity <10,000 mPa·s (10,000 cP) is defined as heavy oil. The latter is further segregated into heavy oil with an API gravity between 10 and 20° API and extra heavy oil with a gravity <10° API. For the purposes of the sampling described herein, these are the definitions for heavy oil, which are liquids at reservoir temperature, and bitumen that are solid (glass like) substances at reservoir temperatures.
Unconventional reserves are sampled to provide information for determining optimal recovery and design production strategies. There are many reasons why reservoir hydrocarbon samples should be acquired. For heavy oil and bitumen, samples are important because they can be used to evaluate production strategies and select the most energy efficient and environmentally acceptable production method. In the case of conventional oil, the sampling process typically extracts a sample with chemical composition and, therefore, physical properties that are representative of the formation fluid. The efficiency of sampling increases with increasing fluid flow-rate Q, which is given by Equation 1 below.Q∝Δp·k/η  Equation 1
In Equation 1, Δp is the pressure difference applied by the sampling tool to withdraw the hydrocarbon, η is the hydrocarbon viscosity and k is the permeability. According to Equation 1, Q increases by increasing either Δp or k and by decreasing η. In practice, the magnitude of Δp is limited by the nature of the consolidation of the formation, and the maximum value of Δp is that at which the formation collapses. Unconventional resources are often found in unconsolidated formations and this imposes another constraint on the sampling method because the borehole collapses when a large differential pressure is applied, which may be necessary during sampling of low mobility fluid. Formation testers are able to achieve pressures drops of about 50 MPa below formation pressure and often require sand filters. Other than fracturing the formation, there is little that can be done to vary k. As a result, sampling methods often rely on methods of decreasing the viscosity η.
There are numerous methods that can be used to decrease the hydrocarbon viscosity as well as to retrieve a chemically representative sample. Obtaining a chemically representative sample may prevent the application of many production methods to sampling because production typically requires the harvesting of energy. The exact chemical composition and physical properties of the formation hydrocarbon can and, indeed in the case of heavy oil and bitumen, have to be altered to produce it in an economically viable manner. In contrast, formation hydrocarbon obtained by sampling may cost orders of magnitude more than the market value of the quantity of formation hydrocarbon extracted.